In the Indies he became acquainted with the ancient church office of deaconess while spending time among the Mennonites, who had revived the institution in 1745.[3] In England he met with English social reformer, Elizabeth Fry, who demonstrated her work among her nation's impoverished and imprisoned people. He returned home not only with a large financial collection for his municipality but also with new ideas about social work among the disadvantaged. He began by working among inmates at the Düsseldorf Prison, preaching the Gospel and ministering to spiritual and physical needs. He walked to and from Düsseldorf every other Sunday until a regular prison chaplain was appointed. The German prisons were then in a very bad state; but those interested in their improvement banded together,[5] and in 1826, Fliedner created the Rhenish-Westphalian Prison Society (Rheinisch-Westfälische Gefängnisgesellschaft). Fliedner realized that the first step must be toward looking after the prisoners on their release, and accordingly, in 1833, he opened at Kaiserswerth a refuge for discharged female convicts.[5] To better support and teach Kaiserwerth's children, he founded a school in 1835 which became the venue for a women teachers' seminar.[6]

In many cities, there were no hospitals at that time. Following somewhat the model of the early Christian Church'sdiaconate, incorporating ideas learned from Fry and the Mennonites, and applying his own thoughts, Fliedner developed a plan whereby young women would find and care for the needy sick. For this, he needed to create Kaiserswerther Diakonie, an institute where women could learn both theology and nursing skills. He opened the hospital and deaconess training center in Kaiserswerth on 13 October 1836. Gertrud Reichardt was the first deaconess commissioned by the new school. Florence Nightingale trained there as a nurse in 1850.[3] One of the associated Kaiserwerth professional schools was later named in her honor. Another noted student was the Swedish Maria Cederschiöld, a pioneer of nursing in her country.[7]

After his wife, Friederike, died in 1842, he found a new life companion (and important employee) in Caroline Bertheau. They opened institutes for the diaconate in 1844 in Dortmund and in 1847 in Berlin with the support of King Frederick William IV of Prussia, and his wife Queen Elizabeth.[5] Fliedner's attention became completely focused on this aspect of the ministry and in 1849 he turned completely to working with the diaconate, including increasing activity abroad. Fliedner's movement has been cited as the model for the Inner Mission movement which Johann Hinrich Wichern developed.[8]

Because of these efforts, deaconess institutes arose in Paris, Oslo, Strasbourg, Utrecht, and elsewhere. By the time of his death in 1864, there were 30 motherhouses and 1,600 deaconesses worldwide. By the middle of the 20th century, there were over 35,000 deaconesses serving in parishes, schools, hospitals, and prisons throughout the world.[9]

A sign of the international respect Fliedner garnered is that his most famous pupil came from outside Germany. English nursing reformer Florence Nightingale first visited in 1841. She was impressed by the religious devotion and noted most of the deaconesses were of peasant origin. She graduated from the facility in 1851. Today, one of Düsseldorf's hospitals (Florence-Nightingale-Krankenhaus) bears her name. [10][11]